LOL BRIEFCASE is just a random thing that people yell because it's funny.

The origin is not certain, but the furthest I've been able to trace it is to a person named PsychicMaster on Warcraft III (Azeroth). He came into the Clan GS channel randomly and just yelled it. Then he'd leave.

PsychicMaster has joined the channel Clan GS.
PsychicMaster: LOL BRIEFCASE
PsychicMaster has left the channel.